Osteoporosis is a major and growing US heath concern. Approximately 44 million Americans currently have osteoporosis or low bone mass and are at an increased risk of a fragility fracture. One out of every two female veterans and one out of every four male veterans will have an osteoporosis-related fracture in their lifetime. Based on the ratio of male to female veterans (12 to 1), nearly six times as many male veterans as female veterans can be expected to sustain a fragility fracture in their lifetime. Maintaining a lifestyle that includes adequate daily weight-bearing activity is the most common recommendation for reducing the risk of osteoporosis. In a 2004 report on bone health and osteoporosis the US Surgeon General concluded that the evidence does not lead to a specific set of exercises or practices for maintaining skeletal health. Thus, the amount, type and intensity of physical activity for maintaining a healthy skeleton is not known. Animal studies in which all aspects of the external loading can be methodically controlled offer the most robust approach for understanding the complex mechanobiological processes that control bone remodeling and skeletal adaptation. The application of external loads to the bones of living animals has provided a wealth of fundamental information about how bone adapts to daily mechanical loading. There remain critical gaps, however, in our knowledge of the relationships among the various parameters that define any loading or exercise protocol (e.g., number of daily loading cycles, loading magnitude, loading frequency) and their influence on the bone adaptive response. The objective of the proposed study is to better understand the relationships among key parameters of exogenous loading protocols that modulate bone functional adaptation. An important concern with previous animal studies with exogenous loading is that the results from different studies are often difficult to compare because of the many different combinations of load cycles per day and applied loading or strain magnitudes. Since bone formation in such studies is known to increase with increasing number of cycles per day and increasing strain magnitude, different combinations of these two loading parameters can yield comparable results, thus making it difficult or impossible to determine the relative contribution of each parameter. Recently, our research group has shown that multiple animal studies with seemingly disparate results can be unified using a single parameter called the daily strain stimulus (DSS), which combines the number of daily load cycles and the strain magnitude using a weighting exponent that determines the relative contribution of each to the total bone stimulus. The proposed study will use this unifying approach to analyze the results of a comprehensive and systematic set of experiments to study exogenous loading and bone adaptation in a mouse model. Sixteen-week old female mice will be exposed to non-invasive, exogenous loading of the right forelimb. Loading protocols will be quantified in terms of the daily strain stimulus. Five experimental protocols will be examined. In the first experiment, the exogenous loading will be applied for 100 cycles per day applied at a frequency of 2 Hz using five different load magnitudes that span the target range of DSS values. The second protocol is designed to duplicate selected DSS values from protocol 1, but with markedly different combinations of cycles per day and strain magnitude to test for the expected duality between cycles and strain magnitude. The third protocol will study the effect of a 10s rest period inserted between each loading cycle. The fourth protocol will study the effect of loading at 10 Hz. The final protocol will combine the 10 Hz loading with the rest insertion design. In all cases, we expect that the DSS will be a strong predictor of bone adaptation. The proposed study will add to our knowledge of the biomechanical factors that affect skeletal health. The results of this research have the potential to influence the design of future exercise intervention programs to combat osteoporosis in humans.